build: simplify Docker builds

[wpf500]

This is my attempt to understand and simplify the new Docker builds in the monorepo. It also fixes the problem that the .dockerignore files were being ignored (haha), as it needs to be in the root of the context.

Mostly it reduces the number of lines needed to add the common package and removes as much context specific knowledge as possible.

[JumpLink]

@wpf500 It's good to know that the .dockerignore simply has to be placed in the root / where the context is. It can then be kept general enough to be used for all docker images. To do this, it would make sense to align the structure of the packages so that, for example, they all use the output path dist. For the backend, it is still built on main (I changed this in my PR with the same goal, but the .dockerignore files are still duplicated and would need to be removed in the package / app pathes)